1. Field of the Invention
The present invention relates to a structure of an alternating current sensor comprising a transformer with two units of secondary winding for measuring the values of alternating current with high accuracy.
2. Description of the Prior Art
Current measurement is an essential work in many electricity applications such as instrument and power system. For example, in the field of electrical power system, the stage of the currents flowing in the systems should be measured from time to time in order to supervise the state of all the equipment connected with the systems and assure their normal operation, therefore any malfunction of the equipment can be counteracted by appropriate protecting means. In the field of the integrated circuit, there are numerous control and data signals being transmitted among the electronic components, if only one of those current signals is erroneously detected, it may result in a catastrophic break down of the entire system. Therefore, if the current in those application can be sensed accurately and reliably, the performance of those application can be substantially improved.
Conventional alternating current sensor can be divided into two categories, one of them is called resistance detecting method. Referring to the circuit in FIG. 1, a sense resistor is connected in series with the circuit whose current is to be measured. After reading the voltage value across the sense resistor, the current to be measured on the circuit can be determined according to Ohm Law with the formula I.sub.in =V.sub.out /R. Although the construction of the instrument is simple, yet the variation of the sense resistance value caused by the thermal effect (P=I.sub.in.sup.2 R) resulted from the current flowing thereon may deteriorate the measurement, especially in the case of large current measurement. Besides, the power consumed by the sense resistor has a influence on the performance of the circuit under measurement.
Another measuring method is by using a current transformer. Referring to the circuit shown in FIG. 2, a kind of instrument transformer usually called a current transformer is employed. The current transformer consists of a primary winding (input current winding) which is connected in series with the circuit to be measured, and a secondary winding (output current winding) which delivers the output current (I.sub.out) proportional to the input current (I.sub.in) with a proportional factor equal to the turn ratio of the two windings. That is, I.sub.in =I.sub.out .times.(M/N), where M and N represent the number of turns of the secondary winding and primary winding respectively. After the current I.sub.out is measured, the current to be measured can be calculated according to above formula. However, voltage is a quantity easier to deal with than current in measurement aspect, it is a general practice to connect a sense resistor R as a load to the secondary winding to measure the value of voltage V.sub.out across the sense resistor. Further, referring to the circuit shown in FIG. 3, the value of the current to be measured I.sub.in can be calculated with the formula I.sub.in =(V.sub.out /R).times.(M/N).
In the above described current measuring method by current transformer, the current to be measured is scaled to a smaller value through a current transformer with suitable turn ratio. With a smaller current flowing in the sense resistor, the power consumption and the thermal effect will be smaller than that of the above mentioned resistance method. Therefore the variation of the value of resistance due to thermal effect will be decreased, and the measuring will be more accurate. However this method still has the following serious problems:
1. There is still necessary to connect a sense resistor to the output winding, and it is inevitable to cause the temperature rise which in turn entails the variation of the resistance value. Such a phenomenon is of great disadvantage to the measurement requiring high degree of accuracy. PA0 2. Due to the fact that the current transformer has its inherent impedance (winding resistance, equivalent core resistance, leakage reactance caused by magnetic leakage fluxes of the windings and magnetizing reactance of the windings), it will produce a voltage drop when current passes it through. Therefore, the value of current converted from output voltage Vout still deviates form its accurate value. PA0 3. Referring to the equivalent circuit shown in FIG. 4, the winding resistance R2 of the secondary winding M generally has a large temperature coefficient of resistance. Therefore, no matter how excellent the quality of the sense resistance R is, the measuring accuracy will be deteriorated by the winding resistance R2 connected in series with it. PA0 4. The winding conductor is in general made of copper material and forms a heterojunction with the sense resistor R, this heterojunction will produce potential work function which interferes the result of measurement. Furthermore, a contact resistance R produced at the junction is also of great temperature dependence, which also deteriorates the measurement. PA0 5. In case the current to be measured is very large, the current ratio, and accordingly the turn ratio of the two windings of the current transformer used for the measurement will also be very large. This causes the difficulty in manufacturing and a narrowing in bandwidth. For one of the solutions, several units of current transformers connected in cascade for scaling down the values of current step by step may be feasible. Referring to the circuit shown in FIG. 5, the voltage V.sub.out is obtained from the secondary winding of the last current transformer by the sense resistor R connected to it, and the current to be measured can be determined by the output voltage and the product of every turn ratio. However, in the above cascade scheme, those undesired effects, such as winding resistance, contact resistance and the thermal effect will also be magnified even thought the current is scaled down. PA0 1. According to the present invention, the measuring winding for picking up the output voltage is in the state of open circuit, and accordingly it conduct nearly no current. Effectively, the output current winding, made of material with small temperature coefficient of resistance, can serve as the load resistance at the secondary winding side. Therefore, the disadvantages caused by thermal effect can be minimized. In addition, the output current winding connects to no sense resistor, therefore the material is homogeneous all over and no heterojunction work function and contact resistance are formed. PA0 2. The voltage induced in the output current winding can be related to that induced in the measuring winding by their turn ratio. Accordingly, the induced voltage of the output current winding can be determined from the voltage picked up at the measuring winding. Because the measuring winding will be connected to a voltage meter with a very large input impedance, there is few amount of current flowing through the measuring winding. The voltage drop caused by the impedance of the iron core and the winding can be neglected, accordingly the loading effect occurred in the conventional transformer can be minimized. PA0 3. The output current winding exhausts nearly the total power consumption for it conducts almost the entire current flowing in the secondary winding side. Because the output current winding does not employ a sense resistor and accordingly has a much smaller resistance value, its power consumption is far smaller compared to that produces in the conventional resistance method or current transformer method. PA0 4. In case the current to be measured is very large, the current flowing in the secondary winding will also be very large if the turn ratio of primary winding to secondary winding does not increase. As to a conventional current transformer which does not utilize the cascade scheme, the sense resistor is unable to withstand such a large amount of heating power generated and may burn up, or assuming the sense resistor is able to withstand such a large heating power, the voltage drop on the resistance will be so large that it is not only difficult to measure but also is dangerous. On the contrary, for the sensor of the present invention, the resistance of the output current winding is very small for it does not connect to a sense resistor, and its thermal power generated and voltage drop induced on the winding are much smaller than those of a conventional device. Even though the voltage induced in the output current winding is extremely high due to a very large current to be measured, the voltage can be read from the measuring winding, and this induced voltage can be decreased by changing the turn ratio of the two set of secondary windings. Besides, the voltage induced in the measuring winding can be decreased by decreasing the resistance of the output current winding, for example, increasing the cross section of the winding conductor used. Therefore, the application of cascade connection of the current transformers is not necessary in case of using the device of the invention to measure large current, the only recommended procedure is to change the turn ratio of the two secondary windings or the value of resistance of the output current winding. The alternating current sensor of the present invention has the advantages of versatility, cost effectiveness and compactness.